# ATOMIC_VAR_INIT

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
#define ATOMIC_VAR_INIT(value) /* implementation-defined */ |  | (since C++11) (deprecated in C++20)

Expands to an expression which can be used to initialize an std::atomic object that can be initialized from `value`. If the atomic object has static storage duration, this initialization is constant initialization.

### Notes

Accessing the variable during initialization from another thread, even through an atomic operation, is a data race (it may happen if the address is immediately passed to another thread with a std::memory_order_relaxed operation)

This macro is primarily provided for compatibility with C; it behaves the same as the constructor of std::atomic.

### Example

Run this code

    #include <atomic>
    #include <iostream>

    int main()
    {
        std::atomic<int> a = ATOMIC_VAR_INIT(1);
        // std::atomic<int> a(1);   // C++-only alternative
        std::cout << "Initialized std::atomic<int> as: " << a << '\n';
    }

Output:

    Initialized std::atomic<int> as: 1

### See also

atomic_init (C++11)(deprecated in C++20) | non-atomic initialization of a default-constructed atomic object (function template)
(constructor) | constructs an atomic object (public member function of `std::atomic<T>`)
C documentation for ATOMIC_VAR_INIT

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